A stopped flow microcalorimeter capable of measuring first order reaction rates of 5 x 10 to the 3rd power per second and temperature changes of 0.0005 degrees C has been developed. This calorimeter is adiabatic to 2% for 2 sec., returns to equilibrium in less than 5 minutes and uses 100 ul of each solution per reaction. It utilizes a thermistor with 3 msec rise time. A mixer has been developed that gives a temperature rise of less than 2 x 10 to the minus 3 power C due to mixing heat at a flow velocity of of 2 m/sec. Computer programs using the finite element technique to simulate the thermal properties of the calorimeter, correct for heat losses, and finite thermistor response time have been developed. A liquid dye laser flash photolysis system producing a 1 microsecond duration output has been utilized in an instrument for the study of oxygen and carbon monoxide binding to human hemoglobin. Interpretation of the observed kinetics in terms of a two state Monod Wyman, and Changeux type model has been analyzed and simulated on the Decsystem 10 using a finite element simulation approach. Equilibrium as well as kinetic information is obtained. Extension of the detector system for use with red cell suspensions is under way utilizing dual wave-lengths at 660 nm and 900 nm. This instrument has also been modified to allow the use of combined stopped flow-flash photolysis.